A force-transmitting mechanism is often used in force-measuring devices, in particular balances of the type in which the principle of electromagnetic force compensation is employed to convert a force originating from a load on a weighing pan into an electrical signal. The force-transmitting mechanism has the purpose to reduce the weight force caused by the applied load, for example in a balance, to such an extent that the resultant reduced force can be converted to a measuring signal whose magnitude is matched to the available load range of a force-measuring device. It is a known fact that in this kind of a force-measuring device based on the principle of electromagnetic force compensation, the displacement of the levers and, accordingly, the deflection of the associated flexure joints are very small.
A force-measuring device of the foregoing description has a parallel-guiding linkage with a stationary portion formed by one parallelogram leg on which a vertically displaceable parallelogram leg is constrained by way of two parallelogram guide members. The force-measuring device further comprises a lever mechanism that is coupled to the parallelogram linkage through a coupling element that is stiff in the lengthwise direction but bends elastically, serving to introduce the force from the parallelogram linkage into the lever mechanism. The force-reducing lever mechanism comprises at least one lever and is supported on the stationary part of the parallelogram linkage. A coupling element is predominantly—but not necessarily—delimited at both ends by a flexure joint consisting of a thin material connection and defining a point of force application. If the lever mechanism has a plurality of levers, the lever arms of levers that follow each other in the lever chain are connected to each other by coupling elements and supported either on the stationary part of the parallelogram or on a preceding lever by a flexure fulcrum comprising a thin material connection.
Every point of a movable part of the parallel-guiding mechanism moves essentially in a plane, a so-called parallelogram plane. The term “the parallelogram plane” as used hereinafter always represents any of the planes in which the points of a parallelogram linkage move.
In U.S. Pat. No. 5,340,951, a commonly-owned patent in which the present inventor is a co-inventor, a force-measuring device of this type is described with a force-transmitting mechanism wherein the lever mechanism has at least one reduction lever being supported on a stationary part, wherein the at least one coupling element is stiff in the lengthwise direction but bends elastically, and wherein the lever mechanism and the at least one coupling element are made of a monolithic material block in which material-free areas are formed. The material-free areas are designed as thin linear cuts separating portions of the material block. The thin linear cuts, which are preferably produced by means of spark erosion, run through the material block perpendicular to the parallelogram plane. The material portion that forms the at least one lever is connected to the material portion that forms the stationary part through a thin flexure joint that forms a lever fulcrum, and it is connected to the coupling element through a locally confined juncture where the force is introduced into the lever.
A weighing transducer based on the principle of electromagnetic force compensation is disclosed in EP 1 054 242 A1, wherein the essential parts, i.e., the parallelogram linkage, the lever mechanism, the coupling elements and the fulcrums, are formed out of a single material block, with a housing-mounted base portion of the material block extending into the space between the two parallelogram guide members and forming a fulcrum support for a first lever. At least one lever is divided at least partially into two levers and at least one coupling element is divided into two coupling element parts that are arranged symmetrically on both sides of a cantilevered part of the housing-mounted base portion.
The shaping of the individual portions of the force-transmitting mechanism out of a single material block can be accomplished either by milling or through an erosion process. It is also conceivable to produce an at least partially monolithic force-transmitting mechanism of this kind through a casting process or an extrusion process.
As already disclosed in U.S. Pat. No. 5,340,951, the coupling element connecting the movable parallelogram leg to the first lever of the lever mechanism has grooves that are cut from both sides perpendicular to the parallelogram plane and are arranged in the mid-portion between the flexure joints of the coupling element, whereby the material thickness of the coupling element is reduced in the area of the grooves. Due to the resultant flexibility of the coupling element in the direction transverse to the parallelogram plane, a possible slight twisting of the movable parallelogram leg which may arise from an off-centered introduction of the force to be measured can be absorbed by the coupling element so that the effect of the twisting transmitted to the lever mechanism and to a connected measuring transducer is reduced.
According to U.S. Pat. No. 5,641,948, a commonly-owned patent in which the present inventor is the inventor, the effect that an off-centered measuring force has on the parallel-guiding mechanism can be suppressed even more strongly through a design in which the area of transverse flexibility of the coupling element is spatially correlated to the axis of the transverse twisting of the parallel-guiding mechanism under a laterally off-centered load, in particular through an arrangement where the portion of the coupling element that flexes transversely to the parallelogram plane is made to coincide with the rotary axis of the transverse twisting of the movable parallelogram leg which occurs under an eccentric load.
The measures described above for suppressing the introduction of lateral forces into the force-transmitting mechanism, which can occur when the force-transmitting mechanism is used in a balance and the weighing load is placed off center, have the disadvantage that a total suppression cannot be achieved in high-resolution force-measuring devices and that transverse forces—even though they are small—are transmitted from the coupling element to the lever system, with the result that a so-called corner-load error can be observed in the measuring result.
Consequently, there is a need for an improved suppression of these forces that are introduced laterally into a force-transmitting mechanism, in particular if the force-transmitting mechanism is used in a high-resolution balance.